Almost all Down's syndrome (DS) patients over the age of 35 to 40 years have histologic features of Alzheimer's disease (AD). However, the presence of extrapyramidal features in up to 36% of these patients has no satisfactory pathologic explanation. We report an older patient with DS, dementia, and parkinsonian signs who showed pathologic changes of Parkinson's disease and cortical Lewy bodies in addition to AD. These parkinsonian changes may be related to chromosome 21 abnormalities.